Triggered by Rain, Semeru's Lava Flood Reaches Gladak Perak

Lahar floods have occurred again in the rivers originating from Mount Semeru, Lumajang, East Java, on Saturday (22/11/2025). Often referred to as secondary hazards, the impacts triggered remain dangerous.

From the video recorded and sent by the Geological Agency, rain-induced lahar floods have been flowing in the Lanang River and Besuk Kobokan. The incident was recorded at around 10:50 AM WIB.

The Kobokan River flows past the Gladak Perak Bridge, which was exposed to hot clouds three days prior. The distance from the eruption's center to the Gladak Perak Bridge is approximately 13 kilometers.

This incident is the second occurrence in the last two days. Previously, lahar flooding also took place in Besuk Kobokan in the Candipuro District, Lumajang, on Friday (21/11/2025).

This resulted in the formation of thick, blackish-gray smoke under pressure. A rumbling sound and thick ashfall accompanied the smoke's movement around the site. (Kompas, 11/22/2025).

As a result, the structure of the Gladak Perak Bridge spanning over Besuk Kobokan appears to be shrouded in smoke, with very limited visibility.

This caused traffic from Malang to Lumajang and vice versa to be temporarily obstructed. Vehicles were waiting on both the left and right sides of the bridge until visibility returned to normal in the evening.

The Head of the Emergency and Logistics Division of the East Java Regional Disaster Management Agency (BPBD Jatim), Satrio Nurseno, confirmed the occurrence of the rain lahar. The vibrations on the monitoring equipment reached a maximum acceleration of 43 millimeters.

On Saturday afternoon, visuals of Mount Semeru were reported to be covered again. Moderate to heavy rain is also falling around Semeru. The rain is prone to triggering lahar floods and hot clouds.

In this regard, the Geological Agency issued a recommendation that residents in and around the river basin be careful and increase their vigilance.

This is in line with the recommendations previously issued by the Head of the Geological Agency, Muhammad Wafid, amid the increasing activity of Semeru. After the eruption, lahar floods are prone to occur, especially along Besuk Kobokan, Besuk Bang, Besuk Kembar, and Besuk Sat.

"We must also be wary of the potential for lava flows in the small rivers that are tributaries of the Besuk Kobokan River," Wafid said.

In addition, the public is urged not to engage in activities within 500 meters from the riverbank along Besuk Kobokan. This area is at risk of being affected by the expansion of hot clouds and lahar flows up to 13 kilometers from the summit.

Citing the website of the Ministry of Energy and Mineral Resources, lahar rain occurs when loose material deposits resulting from volcanic eruptions, which are settled on the summit and slopes, are carried away by rain or surface water.

This lahar consists of a very thick mudflow that can transport materials of various sizes. Large boulders with a diameter of more than 5 meters can float on this mudflow. Lahar can also alter the topography of the river it traverses and damage infrastructure.

According to volcanology expert Surono, there are at least two triggers for lahar floods, namely fine volcanic materials (dust, fine gravel, and sand) as well as deposits of hot clouds. Both are prone to wash away during heavy rainfall.

Large amounts of fine volcanic material can trigger lava floods if they enter rivers and mix with water.

Once it enters the river, the fine material will mix with the water so that the specific gravity of the river water increases and it becomes more powerful. 

It is not surprising that river materials, such as large rocks, are very easily lifted and swept away. This was also observed at Besuk Kobokan on Saturday.

The strength to lift large stones is also influenced by the high slippery nature of water mixed with volcanic ash. Steep contours can cause the flow of lava to be more rapid and highly destructive.

The second cause is volcanic deposits from hot cloud activity. Visually, the volcanic material appears solidified on the outside, but in reality, the interior remains hot, with temperatures exceeding 400 degrees Celsius.

The indications are that there is a sulfur smell and small eruptions due to the deposits of hot clouds meeting with the cold river water. This is evident in the video recordings at Kali Lanang.

Rain can even trigger eruptions, as occurred in 2021. At that time, rain poured over the dome and caused cracks.

Subsequently, the open dome emitted gas, fine and coarse materials, gravel, and boulders that rolled down the slope, forming a pyroclastic flow cloud.

This condition could be more dangerous. The reason is that the hot clouds not arising from volcanic activity are vulnerable to not being recorded by volcanic monitoring equipment.

In the future, mitigation will be a shared responsibility. The potential dangers arising from the increased activity of Semeru do not occur only once during an eruption. Other impacts must also be anticipated from an early stage.